This invention relates to a method of arbitrating access to a data bus, wherein the bus devices are linked by at least one arbitration ring. The invention also relates to a corresponding bus device and to a communication system.
A synchronous, clocked communication system having equidistant properties is a system of at least two subscribers or bus devices that are interconnected via a data network for the purpose of mutual data exchange and/or data transmission. The data exchange occurs cyclically in equidistant communication cycles that are predetermined by the communication clock pulse of the system. The bus devices include, for example, central automation equipment; programming devices; planning and design devices or operator devices; peripheral devices such as input/output modules, drives, actuators, and sensors; and programmable controllers or other control units, computers or machines that exchange electronic data with other machines and, in particular, process data from other machines. Bus devices are also known as network nodes or simply nodes. Hereinafter, the term “control units” refers to regulating units and control units of all types, but also, for example, to switches and/or switch controllers. The data networks include, for example, bus systems such as field bus, Profibus, Ethernet, Industrial Ethernet, FireWire or PC-internal bus systems (PCI), etc., in particular isochronous real-time Ethernet.
Data networks permit communication among a plurality of bus devices through networking, i.e., through interconnecting individual bus devices. Therein, the term “communication” refers to the transmission of data between the bus devices. The data to be transmitted is sent in the form of data telegrams, i.e., the data is bundled into multiple packets and sent, in this form, over the data network to the respective receiver. These packets are therefore also known as data packets. The term “data transmission” is used herein as being completely synonymous with the above-mentioned transmission of data telegrams or data packets.
In distributed automation systems, e.g., in the field of drive technology, certain data must arrive at certain times at the intended bus devices and be processed by the recipients. This is called real-time-critical data and/or data traffic because, if the data does not arrive at the destination in time, unwanted results occur at the bus device.
This is in contrast to data communication that is not real-time critical, e.g., Internet-based or Intranet-based data communication. According to IEC 61491, EN 61491, SERCOS interface—Brief Technical Description (http://www.sercos.de/pdf/sercos_kurzbeschreibung_de—2002.pdf), successful real-time-critical data traffic of the above-mentioned type is ensured in distributed automation systems.
Today, automation components (e.g., controls, drives, . . . ) generally have an interface to a cyclically clocked communication system. An operation level of the automation component (fast cycle) (e.g., position regulation in a control, torque regulation of a drive) is synchronized with the communication cycle. This defines the communication clock pulse. Other, low-performing algorithms (slow cycle) (e.g., temperature regulations) of the automation components may also communicate with other components (e.g., binary switches for fans, pumps, . . . ) only via this communication clock pulse, although a slower cycle would be sufficient. Due to using only one communication clock pulse for transmitting all information in the system, there are high demands on the bandwidth of the transmission link.
The related art discloses various centralized and decentralized arbitration methods for accessing a data bus. In the centralized arbitration method, a central arbiter decides which of the bus devices that have made an access request will access the bus. An example of a centralized arbitration method is the Round-Robin algorithm.
In the decentralized arbitration method, the bus arbitration is performed decentrally in each bus device. An example of such a method is the Daisy-Chain algorithm.